Purification of caustic soda and production of caustic compound



April 1', 1947. D. 5 SMITH Filed May 16, 1942 l MAKE UP N OH LIQUOR H 0H (53 /0 N QOH 8 IM PURITIES) PURIFICATION OF CAUSTIC SODAAND PRODUCTION OF CAUSTIC COMPOUND PURIlFlED acouccnrnneo NoOH,

P H 0 v (cu on) (CH CH) (CH3OH) I PARTIAL ABSORBER PARTIAL CONDENSER CONDENSER conozusan VACUUM T PUMP CO0LER HEATER suoougi ggu pg I MOTHER CRYSTALS LIQUOR Y --1 T- v &

r MELTING BAROMETRH TANK "l'll CONDENSER STRIPPED MOTHER |o- UOR TO EvAPoRAToR(45"/-) a '3 a steam INVENTOR.

= DAVID ESMITH 'ATTORNEYS Patented Apr. 1, 1947 7 cmrso STATES PATENT OFFICE PURIFICATION OF CAUS'I'IC SODA AND PRO- DUCTION OF CAUSTIO COMPOUND David F. Smith, Grosse Ile, Micln, assi nor, by mesne assignments, to Wyandotte Chemicals Corporation, Wyandotte, Mich, a corporation of Michigan as Application May it. 1942," se1-uu No. 443,23: 7

13 Claims. (01. 23-184) 2 The present invention relates to a processfor impurities, cost of operation, of materials and purifying caustic soda, 1. e., sodium hydroxide, reagents required, or relatively expensive and and as an interdependent feature of such proccomplicated equipment. ess, the production of a new chemical composi- I have made the discovery that the treatment tion in the form of a new caustic alkali compound. 5 of caustic soda with an alcohol, e. g., methyl, The caustic soda purifyingprocess of my inventhyl. pr p butyl, amyl and yl cohol tlon simultaneously involves the concentration results in the formation of a solid compound of the sodium hydroxide content of a caustic whose constituents are the alcohol and sodium soda solution, or alternatively, the reduction of hydroxide, and that the latter may be precipithe water content of such solution. tated, the impurities remaining behind, and rel- It is therefore the general object and nature or atively pure sodium hydroxide then recovered by my invention to provide a process, involving the simple decomposition of the alcohol-sodium hyproduction of a new caustic alkali compound, droxide compound. The alcohol recovered from for separating the chemical compound NaOI-I this latter decomposition is then available for from other chemical compounds and elements. re-circuiation and re-use in the treatment of including water, for the purpose of increasing additional caustic soda for purification. the chemical purity of the caustic soda product. So far as I am aware, I am the first to dis- The purification of caustic soda has consticover that an organic hydroxy compound is ca-- tuted a major problem confronting the alkali pable of combining with aqueous sodium hydrox industry for many years. In the commercial go ide and forminga combined compound therewith manufacture of caustic soda, several other chemwhich can be easily isolated as a solid and from ical elements and compounds become necessarily which sodium hydroxide can be recovered by included in the product. Their presence is unsimple decomposition. A solid compound dedesired and, hence, they have generally been rived by crystallization from a mixture of abtermed fimpurities." Such additional chemical solute alcohol and anhydrous caustic soda has compounds and elements, constituting such impreviously been reported in the literature. Howpurities, are, for example, sodium chloride, soever, theformation of solid compounds from dium chlorate (particularly prominent in caustic water solutions of NaOH with alcohols to prosoda made by the diaphragm electrolytic cell' duce a molecular combination .of the sodium hyprocess), sodium carbonate, iron, manganese and 3 droxlde and the alcohol is novel. In. particular, silica. Sincelarge quantities of caustic soda are th treatment of caustic soda solutions of apsold, transported and used in their more highly proximately by wei'ght'NaOH content with concentrated form, viz., 01' by weight of an alcohol selected from the group of methyl, NaOH content or greater, the reduction of the ethyl, propyl, amyl, butyl and benzyl alcohols,

water content of caustic soda solutions, during 35 u h l ohol being add d i n amount con- 7 the process of manufactm'e, has also been a desiderably less than the weight of the sodium hysired objec iv of he in y. a hence. n droxide present, e. 3., in not over an aliquot the broad consideration of my present invention, fraction, thereof, results in the precipita ion from water is also to be regarded as an impurity, th liquid mixture of new solid compounds whose falling Within e eneral obiective of i 4o com osition is made u of combined molecules ing the relative percentage content of the chemof sodium hydroxide and of th alcohol. ical CQmPmmd NaoH the 5x01115101! P other When methyl alcohol is employed as the alcochemical compoun an elements initially hol for treating caustic soda, a new and useful tra n d in caustic alcohol compound is produced. This new Heretofore, the problem of puriiying caustic 5 compound, whose composition is indicated by soda has been practiced'and attempted in several the chemical formula ,c 0 1s g fi g g s-gs' g fi zg s zg'z ggfg a white crystalline compound, which does not exhibit hygroscopic tendencies as great as those either the NaoH or hydrate thereon or of solid caustic soda itself. but still possesses a purities, has been disclosed. See U. 8. Patents 5 Nos. 971,144, 1,733,879 and 1,865,281. The treathigh degree alkalinity and hence is indicated t as a valuable and desirable detergent material. gigi z f g m g mgf 2x 3 12 333i Additional objects and advantages shall beties has also been known and practiced, involvcome apparent. the f l wing'description proing the use of such reagents as aqueous ammonia 5 ceeds- (U. S. Patent No. 1,961,590); magnesium Tothe accomplishment of .the foregoing and pounds (U. S. Patent No. 1,997,691); and sorelated ends, this invention, then, consists of the dium sulphate (U. S. Patent No. 1,888,886). All features hereinafter fully described and particof these prior processes, however, have involved ularly pointed out in the claims.

certain difllculties and shortcomings, either The annexed drawing and following descripfrom the standpoint of efllciency of removal oi tion set forth in detail a few of the various ways 3 I in which the principle of the invention may be employed. r

The annexed drawing is allow sheet illustrating the'process embodying theprinciple of my invention.

Now referring more particularly to the draw ing, it will be seen that the heavier arrows illustrate the main flow, or principal operative steps of the caustic soda purifying process. Thus, caustic soda liquor on the order 01' 50% NaOH content by weight, viz. 53% NaOH concentration, as noted in the flow sheet, is introduced to the absorber condenser along with methyl alcohol or methanol (CHJOH) The temperature in the absorber condenser is preferably in'the range of 60-70 C. This prevents any high local concentration of methanol which might result in precipitation of salt along with the crystals of the resultant compound. The solution from the absorber condenser is then pumped to the cooler where the crystalline compound of sodium hydroxide, methanol and water, is formed and pre- 4 temperature :120 6., thus reducing the operating temperature at the column.

In the initial treatment 0! the caustic liquor, with methanol, not over by weight of methanol on the basis of the caustic soda solution is required. The methanol is recovered in the corollary operations of the process, and, as shown on the flow sheet, for re-use in treatment of additional caustic liquor for purificatibnj Thus the mother liquor from the centrifuge will contain about 2 to 4% by weight of methanol, and the cipitated out from the mother liquor, the impurities such as NaCl and NaClO: remaining in solution in the mother liquor. The action of the cooler removes the heat of crystallization, thus increasing the yield of crystals of the precipitated line in the flow sheet, these crystals having the chemical formula 3Na0H.2HzO.CI-I:OH, may be removed as a product of the process inand 01.

itself. This new compound is quite stable at atmospheric temperatures and composed of white crystals having a boiling point of approximately 128 C. and a melting point of about 119 C. It is not as hygroscopic as caustic soda and possesses considerable alkaline properties.

As the crystals are separated from the mother liquor in the continuous centrifuge they may be finally washed with a little water, pure caustic soda solution, methanol or pure sodium hydroxide-water-methanol solution of composition corresponding to that of the mother liquor, depending on the degree of purity required in the purified caustic. If especiallypure crystals of the new compound are desired, they may also be washed with a solvent such as ether.

The crystals are then introduced to the melting tank where they are mixed with sufilcient hot purified caustic liquor of '70 to 73% NaOH content, in order to make a slurry which is pumped into the vacuum stripping column. This column contains a few distillation-column plates and a tubular nickel callandria heated by steam. It is in the vacuum stripping column wherein the sodium hydroxide-water-methanol compound is decomposed to form relatively pure sodium hydroxide and methanol. When the crystals with a normal amount of mother liquor are stripped of methanol alone without removing any water, about 72% by weight NaOH remains. Water tends to come out-to some extent with the methanol, and if desired, a partial condenser may be added in conjunction with the stripping column for returning such water thereto. The concentrationof the resultant purified caustic liquor, depending on the amount of water initially present, the amount removed with the methanol and the amount returned, is in the range of 70-73% NaOH content. As noted, the stripp column is operated under a vacuum, induced by the barometric condenser, and the steam introduced at a centrifuged crystals up to 17% by weight of methanol, depending on the completeness of removal of mother liquor from them. r

The methanol is recovered from the mother liquor by passing it throughthe barometric condenser, pumping it through a heater and into the plate column which is operated 'at atmospheric pressure, and heated to 130 C. The methanol vapor emitted at the top of the plate column is then introduced to the absorber. condenser at the beginning of the main process. The mother liquor from the bottom of the plate column, containing the impurities, may then be evaporated to a concentration of about 50-53% NaOH content, cooled andNaCl crystallized therefrom, reducing the content of the latter in the mother liquor to about 1%, thus renderingsuch mother liquor ready to be itself introduced to the main process for purification and concentration, such as by mixing with additional untreated caustic liquor in an amount corresponding to the amount oi sodium hydroxide removed in the form of the sodium hydroidde-water-methanol compound.

The methanol from the crystals is recovered both from the melting tank and from the vacuum stripping column. Thus the methanol vapor from the meltingtankis simply passed through the partial condenser which operates to reflux the water from themethanol vapor and the latter is re-introduced-to. the absorber condenser at the beginning of the process. The methanol and water vapor from the top of the vacuum stripping column are similarly treated in the barometric condenser, and the methanol vapor then returned by the vacuum pump to the absorber condenser. Sufficient make-up methanol may be added to the absorber condenser from time to time to compensate for any minor losses which occur during the re-circulation and re-use of the methanol in the process.

It is to be noted that in the above process, as

tated crystals than in the original mother liquor,

as the crystals form, the mother liquor will become more dilute in NaOH and hence, the NaCl solubility in the mother liquor will increase, and finally the NaOH concentration in the mother liquor will decrease to the point where further additions of methanol will not produce appreciably more crystals. Thus a point of diminishing returns is reached where the addition of methanol merely adds to the amount to be later recovered from the mother liquor.

A limitation of amount of methanol going into the mother liquor to not over 10% by weight is accordingly to be desired under ordinary conditions of operation.

The following example of laboratory test data conducted upon the operation of my above-de- '5 scribed process, will further serve to illustrate the principle of the invention, and will enable those skilled in the art more conveniently to comprehend same: Impure caustic liquor in the amount of 2959 grams containing 52.9% by Weight NaOH, 1.024% NaCl and .0792% NaClO: was heated to 70 C. and 241.7 grams of 98% methanol added ina closed container. The temperature of this mixture under influence of the heat of dilution, rose to 77 C., at which point a homogeneous solution was obtained. In order to obtain equilibrium conditions, this solution was then cooled with stirring to 35 C. over a period of six and one-half hours. consisting of sodium hydroxide, methanol and water started to appear at a temperature of 76 C. The mixture of cooled crystals and mother liquor was centrifuged for about minutes at 2500 R. P. M. in a centrifuge with an 8 in. diameter basket fitted with a nickel gauze screen. The weight of crystals obtained was 854 grams of the following analysis:

Per cent NaOH 62.3 CI-IsOI-I 15 'NaCl .089 NaClOa .0053

Per cent NaOH 44.05 CHsOH 3 NaCl 1.272 NaClOa .102

(97 gramsof crystals, mother liquor and methanol were lost in handling.)

From the data of the foregoing example, con- Crystals of the new compound sisting essentially of laboratory experiments, and

not intended in any way to limit the scope of my invention, but merely for purposes of illustrating the mode of operation and beneficial results thereof, it will be seen that the NaOH content of the caustic soda solution was increased from 52.9 to 73% by weight, representing a 37.8% increase on the basis of the original amount; the NaCl decreased from 1.024 to .089%, representing a 92% decrease upon the basis of original content; and the NaClOa reduced from .0792 to .0053%, representing a 93.5% decrease on the basis of original content.

Thus it will be seen that in addition to the above pointed out advantages of substantially reducing the impurities found in caustic soda, together with the production of a new caustic alkali compound, that the process of my invention also possesses the highly desired economic advantage of reducing the water content of the caustic soda solution, or alternatively stated, increasing the NaOH concentration. This represents anvadditional beneficial result and productive asset of the invention.

The possible build up of the chlorate and iron content in the mother liquor, where the latter is evaporated and re-cycled to the process, can, of course, be eliminated by removal treatments therefor well known to those skilled in the art. However, such corollary removal treatments do not constitute any part of the essential characteristic of novelty of my invention as hereinabove described and as hereinafter particularly pointed out and defined in the claims.

Other modes of applying the principle of my invention may be employed instead of the one explained, change being made as regards the composition and method herein disclosed, provided the ingredients or steps stated by any 01' the following claims or the equivalent of such stated ingredients or steps be employed.

I therefore particularly p int out and distinctly claim as my invention:

1. The method of increasing the sodium hydroxide content of an aqueous caustic soda solution containing sodium hydroxide, water and other chemical compounds, consisting in adding an alcohol to said solution, the amount of the alcohol employed being considerably less than the weight of NaOH, but sufilcient to form a constituent of the combined molecules of NaOI-I and the alcohol, effecting an increase in temperature,

then cooling the solution without removing the to said solutions, the amount of the alcohol being considerably less than the weight of NaOH, but sufiicient to form a constituent of the combined molecules of NaOH and the alcohol forming a solid compound comprising sodium hydroxlde and said alcohol as ingredients thereof without removing thewater, separating said solid compound from the remaining solution, decomposing said solid compound to recover sodium hydroxide and alcohol and thenreturning such alcohol to the process for treatment of additional caustic soda solution.

3. The method of increasing the sodium hydroxide content of caustic soda solutions containing sodium hydroxide,' water and other chemical compounds, consisting in adding an alcohol to said solutions selected from the group consisting of methyl, ethyl, propyl,butyl, amyl and benzyl alcohols, the amount of the alcohol being considerably less than the weight of NaOH,

but sufllcient to form a constituent of the combined molecules of NaOH and' the alcohol, forming a precipitate containing sodium hydroxide and the alcohol without removing the water, separating said precipitate from the remaining solution, and then recovering sodium hydroxide from said precipitate and the alcohol as a byproduct. l

4. The method of increasingthe sodium hydroxide content of caustic soda solutions containing sodium hydroxide;water and other chem ical compounds, consisting in adding an alcohol to said solutions selected from the group consisting of methyl, ethyl, propyl, butyl, amyl and benzyl alcohols, the amount of the alcohol being considerably less than the weight of NaOH, but suflicient to form a constituent oi the combined molecules of NaOH' and the alcohol, forming a solid compound comprising sodium hydroxide and tions, consisting in adding an alcohol to the aqueous solution of caustic soda containing sodium chloride and other impurities, the amount of the alcohol being considerably less than'the weight of NaOH, but sufflcient to form a constituent of the combined molecules of NaOH and the alchol, precipitating a compound comprising sodium hydroxide and said alcohol without removing the water, separating said precipitated compound from the mother liquor of the solu- A tion, removing the alcohol from said compound,

removing the alcohol contained in said mother liquor, returning both portions of such removed alcohol to the process for treatment of additional caustic soda solution, evaporating the mother liquor from which the alcohol has been removed, cooling said mother liquor and removing the excess sodium chloride therefrom, and then mixing the resultant mother liquor with-additional caustic soda solution for recycling through the aforesaid purifying process.

6. The method of purifying caustic soda consisting in treating aqueous caustic soda containing impurities with methanol the amount of methanol being considerably less than the weight of the NaOH present, but sufflcient to form a 8 additional caustic soda solution, evaporating said mother liquor, cooling said imother liquor and removing excess sodium chloride therefrom, and

then mixing the resultant mother liquor with additional caustic soda solution for re-cycling in the aforesaid purifying process.

10. The method of concentrating caustic soda solutions from approximately 50% by weight -to 70-73% NaOH content consisting in mixing methanol with an aqueous caustic soda solution of about 50% NaOH content in an amount considerably less than the weight of NaOH, but sufflcient to form a' ternary compound, forming a precipitate 4 consisting of sodium hydroxide,

methanol and water, separating the precipitate from the mother liquor and then decomposing said precipitate with heat to produce a caustic soda solution of 70-73% NaOH content.

11'. The method of concentrating caustic soda solutions from approximately 50% byweight to 70-73% NaOH content consisting in adding methanol in the amount of not over 10% by weight on the basis of caustic soda solution, but

'suflicient to form a ternary compound, to a ternary compound, thereby forming a solid com- I pound in which sodium hydroxide, water and 'methanol are constituents, separating the solid compound from the mother liquor and then recovering purified caustic soda from said com pound. N

,7. The method of increasing the sodium hydroxide content of a caustic soda solution containing sodium hydroxide, water and other chemical compounds, consisting in mixing methanol with said solution, the amount of methanol bein considerably less than the weight of'the NaOH present, but sufncient to form a ternary compound cooling the solution without removing the water, thereby forming a precipitate com-'- prising a compound in which sodium hydroxide,

water and methanol are the constituents thereof, separating said precipitate from the remaining solution, and then recovering sodium hydroxide from said precipitate.

8. The method of increasing the sodium hydroxide content of a caustic soda solution containing sodium hydroxide, water and other chemical compounds, consisting in adding methanol to said solutionI the amount. of methanol being considerablyless than the weight of NaOH,

' but suflioient to form a ternary compound, coolsufficient to forma ternary compound, precipitating therefrom a compound in which sodium hydroxide, water and methanol are the constituents thereof, separating said compound from the mother liquor of the solution, separating methanol from said compound and from said mother liquor, returning both of the latter portions of said methanol to the process for treatment of causic soda solution of about NaOH content,

= forming a precipitate consisting of sodium hydroxide, methanol and water, separating the precipitate from the mother liquor and then decomposing said precipitate with heat to produce a caustic soda solution of 70-73% NaOH content.

12. The method of purifying caustic soda, consisting in treating aqueous caustic containing impurities with an alcohol and without removing the water, the amount of the alcohol employed being considerably less than the weight of NaOH,

but suflicient to form a constituent of the combined molecules of NaOH and the alcohol, forming a solid compound containing sodium hydroxide an d the alcohol, separating the said solid compound from the mother liquor and thenrecovering relatively pure caustic from the said compound and the alcohol as a by-product.

13. The method of purifying caustic soda,

consisting intreating aqueous caustic soda containing impurities with an alcohol selected from the group consisting of methyl, ethyl, propyl, butyl, amyl and benzyl alcohols, and without re- ,moving the water, the amount of the alcohol employed being considerably less than the weight of NaOH, but sufllcient toform a constituent of the combined molecules of NaOH and the alcohol,

, forming a solid compound containing NaOH and the alcohol, separating the said solid compound from the mother liquor and then recovering relatively pure caustic from the said compound and the alcohol as a bi -product.

' DAVID F. SMITH.

' REFERENCES orran The following referencesare of record in the file of this patent: Y UNITED ST TES PATENTS Number I Name D 2,127,496 Waldeck Aug. 23, 1938 1,712,830 Kyrides May 14, 1929 FOREIGN ra'rnm's Number Country Date British July 21, 1926 255,880 OTHER REFERENCES Holleman et al., Text-Book of Organic Chem-. istry," page 66, first edition (1903). John Wiley 8: Sons, New York. (Copyin Scientific Library.) 

